Field
The present disclosure relates to time-temperature indicators (TTI), and to time-temperature indicators displaying an indication of temperature records over time.
Background
The following show some of the TTI available either in the commercial market or being described in literature. The operational principles of various TTI products are briefly described:
Diffusion-Based TTI
A diffusion-based TTI relies on diffusion of reactive components, in which the diffusion is time and temperature dependent. An example of a diffusion-based TTI is Monitor Mark® TTI, commercialized by the 3 M Company, based on the temperature-dependent diffusion reaction of a colored fatty acid ester along a porous wick made of high-quality blotting paper. Its measurable response is the distance of the advancing diffusion front from the origin (Kerry et al., 2006).
Red-ox Dye
Hu and Loconti (1973) and Gohil (2006) have reported a receptacle that contained a metered amount of oxidized red-ox dye and an effective amount of reducing agent in an alkaline medium, solvent or moisture retainer. The conversion of the oxidized red-ox dye into its reduced form occurs inside the sealed receptacle. The color change can also come from the oxygen diffusing through the barrier film and react with the reduced dye.
Lewis (2002) developed a time indicator that consists of a tissue paper saturated with reduced dye and covered with a plastic cap. The oxygen diffuses through the plastic cap that leads to a color change from the oxidation of the dye. The plastic cap is thick in the center and gradually thins outward toward the edge. This construction provides a gradual color transition from the edge to the center with the elapse of time.
Color Change as a Result of Solid State Polymerization of Substituted Monomers
Polymer-based systems such as the Fresh Check® TTI produced by the company TempTime, are based on the solid state polymerization of a thinly coated colorless acetylenic monomer that changes to a highly colored opaque polymer at a temperature-dependent rate (Nuin et al., 2008).
Based on Time-Temperature Dependence of Enzymatic Reaction (Vitsab®) Leak & Rönnow, 2000
Enzymatic systems such as the VITSAB Check Point® TTI are based on a color change in the TTI induced by a pH drop resulting from the controlled enzymatic hydrolysis of a lipid substrate which changes the color of the chromatic indicator from green over yellow to orange red (Kerry et al., 2006; Tsironi et al., 2008).
A plastic outer compartment contains two mini-pouches; one contains a water solution of lipolytic enzyme, and the other is lipid substrate water solution containing a pH indicator. The TTI is activated by breaking the wall between two mini-pouches and the contents are mixed by outside force. A color change from green to clear yellow is due to the controlled enzymatic hydrolysis of a lipid substrate with a decrease in pH.
Alternative enzymatic systems have been described. Sun et al. (2008) developed a new amylase type TTI based on the reaction between amylase and starch. Bauer and Knorr (2005) reported a pressure induced starch gelation as a pressure time temperature indicator (PTTI).
Solid State Reaction Systems
Solid state reaction systems represented by the OnVu™ TTI produced by the Ciba company are based on photosensitive compounds such as benzylpyridines. Once exposed to a low wavelength light, they become colored and this colored state reverses to the initial colorless state according to temperature (Tsironi et al., 2008). The OnVuÒ (2006) time-temperature indicator from Ciba Specialty Chemicals and Fresh Point relies on the properties of pigments that change color over time and temperature. The OnVuÒ is activated by UV light to first become dark blue and then gradually changes to light color as time passes.
Microbiological TTIs
Microbiological TTIs are proposed by the French company CRYOLOG. TRACEO® and (eO)® are microbiological TTIs made of selected strains of lactic acid bacteria. Prior to utilization, these TTIs are stored in a frozen state (−18° C.) to prevent bacterial growth in the TTI medium. As these TTIs are very thin, their activation is obtained simply by defrosting them for a few minutes at the room temperature. The TTIs are put on the food, and in case of temperature abuse or when the product reaches its “use-by” date, the temperature-dependent growth of the TTI microorganisms causes a pH drop in the tags, thereby leading to an irreversible color change of the medium chromatic indicator which becomes red (Ellouze et al., 2008).